We intend to study factors affecting changes in transmission through a spinal reflex circuit in the mammalian central nervous system. The reflex chosen for investigation is the plantar cushion (PC) reflex, a cutaneous reflex in the cat. The basic neural circuit of the PC reflex appears to consist of not more than two serially connected dorsal horn cells, interposed between the cutaneous afferents and the motoneurons to the intrinsic plantar muscles. Further neurophysiological analysis, employing extra- and intracellular recording of single spinal units, will be undertaken to clarify the circuit components of the PC reflex. Repeated stimulation of the plantar cushion at 2-10 Hz typically leads to enhanced transmission (sensitization) followed by waning (habituation) of the reflex. It appears that during repeated stimulation of PC, major changes in transmission occur in the region of the first-order dorsal horn cells responding to PC stimulation, and that these changes may explain at least some of the changes in transmission of the PC reflex during iterated stimulation. Whether these changes in transmission during repeated stimulation are pre- or postsynaptic, and whether they involve intrinsic changes in the reflex neurons, or modulation via pre- or postsynaptic input from other neurons, will be investigated. Also an attempt will be made to condition the PC reflex. If the PC reflex can be classically conditioned, it will be used to study neural mechanisms related to changes in transmission occurring during conditioning.